ABSTRACT/PROJECT SUMMARY Children with ADHD are at elevated risk for depression in adolescence and young adulthood, and this comorbidity is associated with far greater impairment than either disorder alone, including higher rates of inpatient hospitalization and increased risk for suicidal ideation and behavior. Despite these adverse outcomes, existing evidence-based interventions for ADHD have not demonstrated effects in reducing depressive symptoms nor do established depression prevention programs work as well for adolescents with ADHD. Reward responsivity (RR) and emotion regulation (ER) are two key factors demonstrated to mediate the association between ADHD and depression and are thus key targets for prevention. In an effort to address the gap between access and utilization of mental health care, which is especially stark among adolescents with ADHD, we aim to develop and test a modified behavioral activation prevention program [Behaviorally Enhancing Adolescents? Mood in Schools (BEAM-S)] that incorporates modules to directly target these purported mechanisms. In line with the Deployment-Focused Model of Intervention Development and Testing, we will develop a program that can be easily implemented and sustainable in the high school setting as delivered by school mental health staff that reduces depression and improves overall functioning in adolescents with ADHD, by way of improving RR and ER. As a first step, we will conduct focus groups with community stakeholders (e.g., school mental health providers, school administration, adolescents, parents, teachers) to develop an acceptable and sustainable selected and indicated prevention program for school staff to utilize. We will then initially train school staff to deliver this prevention program via a co- therapy training model to prepare them for their independent implementation of the prevention program in a case series consisting of two groups (one in each of two high schools) to evaluate preliminary feasibility. We will then conduct a stage 1 RCT with a hybrid-type 1 implementation- effectiveness design in which four high schools will be randomized to either the BEAM-S condition (n = 2) or treatment as usual (n = 2). Finally, qualitative and quantitative data from aims 1-3 will be used to prepare final procedures for a future large-scale effectiveness trial R01 (aim 4).